Merosin is a 700 kDa multichain protein that has several properties in common with laminin, one of the major extracellular matrix proteins present in basement membranes. Both contain B1 and B2 light chains, but merosin has a distinct M-chain compared with the laminin A-chain. Merosin is present in the basement membrane of placenta, striated muscle, and peripheral nerve. Using the RT-PCR assay, we demonstrate that merosin, but not laminin, is also expressed in the thymus. (Previous reports of laminin expression in the adult thymus are probably in error). A partial cDNA sequence of the mouse merosin M-chain was 88% identical to the human M-chain, and the deduced amino acid sequences were 95% identical. 18-24% of adult mouse thymocytes bound to human merosin in vitro. Mg2+, but not Ca2+, was essential for binding. Binding was inhibited by antibodies recognizing VLA alpha6 or beta1, suggesting that the VLA-6 integrin is a merosin receptor. An anti-beta4 integrin subunit mAb failed to inhibit binding, suggesting that the alpha6beta4 integrin was not involved. Thymocytes were fractionated into immature and mature populations based on their expression of the heat stable antigen, recognized by the J11d mAb. Virtually all thymocytes expressed VLA-6, but only immature thymocytes (J11d+) bound to merosin. PMA treatment did not significantly increase the binding of J11d+ thymocytes nor did it induce binding in the mature J11d- population. In contrast, both splenic T cells and unseparated lymph node cells showed enhanced binding to merosin after PMA stimulation. The expression of merosin in the thymus and its selective interaction with immature thymocytes suggest that thymocyte-merosin interactions may play a role in T cell development. We have analyzed the expression and function of alpha4beta1 fibronectin binding integrin receptors on human thymocytes. A major portion of double positive (CD4+CD8+) human thymocytes express alpha4beta1 in a constitutively active form and adhere to fibronectin and vascular cell adhesion molecule 1. alpha4beta1 expression is similar on adherent and nonadherent populations, thus, activity reflects the receptor state and not simple expression. These adherent cells are immature, expressing high levels of CD4/CD8 and low levels of CD3 and CD69. In contrast, nonadherent cells possess the phenotype of thymocytes after positive selection, expressing intermediate levels of CD4 and/or CD8 and high levels of CD3 and CD69. The adherent population fails to respond to activation with anti-CD3 and fibronectin, whereas nonadherents exhibit an alpha5beta1-dependent proliferation. Differential regulation alpha4beta1 and alpha5beta1 receptors may provide a mechanism for controlling cellular traffic, differentiation, and positive selection of thymocytes.